Anyone even marginally involved in the building trades has employed a device commonly referred to as a tape measure or more accurately, a spring retractable tape rule assembly, in which a blade, usually configured of a metal ribbon is coiled within a housing and which extends from the housing from a position tangential to its reel outwardly through a spaced opening in the housing assembly. These blades are further generally configured with hooks appended to their extreme ends and bent at a 90° angle for releaseable attachment to a ledge or other surface of a monument in order to measure the distance from it.
When employing the retractable rule assembly as described above in which relatively short distances are measured, such a tool has proven to be accurate, convenient to use and virtually indispensable to anyone having the need to make linear measurements. However, this same convenient tool can be frustrating to use when measured distances are increased. In such circumstances, the low flexural stiffness and torsional stability of the blade limits its maximum standout length. In other words, the metal blade tends to buckle and collapse or roll over from its linear extended orientation making it difficult to extend the blade for taking convenient and accurate length measurements. When this occurs, a user must generally retract the metal blade back within its housing before attempting to achieve blade extension, hopefully, without collapse.
Current retractable rule assemblies have employed blades having concavo-convex cross sections in order to provide some measure of bending stiffness. Although curving the blade cross section does increase its maximum standout length and thus reduces tape buckle and collapse, it does little to prevent rollover, and when distances become large enough, even a curved blade does not prevent such collapse. In addition, the concavo-convex blade cross section often times results in the inability to obtain accurate length measurement readings and introduces errors when attempting to employ demarcations along the blade for distance marking. This convex shape also makes it difficult to releaseably hook the blade to a vertical edge and measure horizontal distances for the convex shape results in the blade's strong tendency to rollover and thus unhook itself. Current blades also are difficult to employ in making a measurement when the blade extremity supporting the hook is pushed against an obstacle such as a corner in order to obtain an inside measurement because the blade buckles and collapses when confronted with a relatively small axial load.
Others have recognized the shortcomings inherent in using single ribbon retractable rule assemblies. For example, in U.S. Pat. No. 3,032,151, it was proposed that a metal beam be configured of two convex metal ribbons welded together at their edges “having an inherent tendency to assume a rectilinear state that is efficiently flexible to permit coiling.” Although the patentees were correct that such a dual ribbon configuration did greatly improve standout and torsional rigidity, the blade disclosed therein did not acceptably coil within the tape measure housing to create a usable commercial product. Specifically, when the dual ribbon blade of the configuration disclosed in the '151 patent is coiled, large compressive stresses and buckling of the inside layer develop. This causes severe distortion of the dual ribbon blade which prevents tight packing of the coils and thus severely limits the length of tape that could be stored in a conventional housing. Also, the method of joining as proposed for this device results in a severally limited fatigue life, further rendering it unsuitable for commercial applications.
Another “solution” to this problem was suggested in U.S. Pat. No. 2,319,724 in which a conventional tape was reinforced by providing a plurality of separate but overlapping metallic plates which, according to the patentee, acted to prevent the tape from buckling or bending so that it could be extended for greatly increased lengths. This product required the reinforcing plates to be individually removed, plate by plate, in coiling the tape within its retractable rule assembly housing. This tedious task made the product commercially unsuitable.
Yet another approach was suggested by U.S. Pat. No. 1,947,392 whereby a hollow rod was proposed being configured of at least two metal bands disengageably coupled with each other along their longitudinal edges and at the same time being at least partially curved in cross section and adapted to be jointly wound in a flat state after disengagement. However, in using such a device, an inordinately large force was required to retract the tape noting that the multi-part blade did not coil compactly within a housing of acceptable dimension unless very short tape lengths were employed.
It is thus an object of the present invention to provide a retractable rule assembly having a blade with improved standout and torsional rigidity to enable the blade to maintain its rigid linear orientation when extended relatively long distances from the assembly housing and easily retracted to a compact coil within the housing.
It is yet a further object of the present invention to provide a retractable rule assembly in which its blade has improved stability enabling one to handle the blade in virtually any orientation at or near maximum standout in push or pull applications.
It is yet a further object of the present invention to provide a retractable rule assembly in which its blade can be easily restored to its beam shape following inadvertent collapse without retracting and reextending the blade as is necessary in current commercial products.
These and other objects will be more readily appreciated when considering the following disclosure and appended claims.